Generic Technology of these flourishing high technology industries such as digital television, new generation mobile communications, broadband communications network and family consuming electronics focuses on multimedia of which the main content is video and audio processing technology, particularly on the data compressing technology. High-efficient video coding/decoding technology is the key of realizing high quality and low cost for storing and transmitting multimedia data. At present, the conventional coding methods include predictive coding, orthogonal transform coding, vector quantization coding, etc. All these methods are based on the signal processing theory, usually called first generation coding technology. The popular international coding standards for images are based on this coding theory which adopts coding method combined of motion compensation based on block matching, discrete cosine transform and quantization. Typically, the first joint technology committee of International Standardization Organization/International Electro-technical Commission (ISO/IEC JTC1) proposes motion picture experts group (namely to MPEG)-1, MPEG-2 and MPEG-4 and such international standards; and the International Telecom Union(ITU-T) proposes the H.26x series. These video coding standards are widely used in the industries.
All these standards for video coding adopt Hybrid Video Coding strategy normally including four main modules such as predicting, transforming, quantizing and information entropy coding. wherein, the function of predicting module is to predict the current image to be coded by using the coded and reconstructed image (inter prediction), or to predict the current image block to be coded by using the coded and reconstructed image block in images (intra prediction); the function of the transforming module is to convert the image block inputted into another space so as to converge the energy of inputted signals at transform coefficient of low frequency for lowering relativity among the elements within the image block, and being useful for compressing; the main function of quantizing module is to map the transformed coefficients into a limited element aggregate advantageous to coding; and the main function of information entropy coding module is to represent the quantized transform coefficient with variable length code according to the statistical principle. The video decoding system has similar modules, of which to reconstruct the decoded image through the procedures of entropy decoding, inverse quantizing, inverse transforming etc. Besides the above modules, the video coding/decoding system usually also includes some assistant coding tools, which dedicate to coding performance (compression ratio) of the whole system.
Most coding efficiency of video coding is from prediction based on motion compensation. The main function of the prediction based on motion compensation is to eliminate redundancy of video series on time. The procedure of the video coding. is to code each frame image of video series which is realized by the prediction module. The conventional video coding system which codes each frame image is based on. image block as a basic unit. When coding each frame image, there are intra coding (I frame), prediction coding (P frame), bi-directional prediction (B frame) coding, etc. Generally, when coding, I frame, P frame and B frame coding are interlarded, for example based on IBBPBBP sequence.
The introduction of B frame may effectively solve occlusion problem caused by different motion directions and motion rate between motion objects or between objects and their background. B frame coding may achieve a bit rate of over 200:1 for coding and compression efficiency. Coding. the image block of B frame includes four modes: direct, forward prediction, backward prediction and bi-directional prediction. Since the B frame technology needs to process forward and backward motion estimation simultaneously, higher computation complexity is needed. At the same time, in order to discriminate forward and backward motion vectors, the extra identification information is needed to introduce into.
In conventional video coding system, B frame usually possesses the motion mode of bi-directional prediction, for which can effectively eliminate inaccuracy of inter prediction caused by the rotation of images, variation of luminance, noise, etc. However, more motion vectors are needed to be coded at same time. Hence, the bit number for coding motion vector in proportion to the whole process of coding is usually more than 30%.
Therefore, if there is a method which can lower coding for motion vectors under the precondition of keeping nice bi-directional prediction performance, it will effectively improve compression ratio of coding especially meaningful for the application of video transmitting at low bit rate and lowering bit number to be needed in coding motion vectors.